1. Field of the Invention
The present invention relates to a Karman vortex flow meter. Particularly, the present invention relates to a Karman vortex flow meter for reducing a pressure loss in fluid passing through it by improving the shape of its flow passage.
2. Discussion of Background
A Karman vortex flow meter is generally of such construction that a detection device for detecting a quantity of a vortex flow is disposed in a tubular main passage. In order to make the manufacturing of the Karman vortex flow meter having different cross-sectional surface areas easy, it has been proposed not to change the cross-sectional surface area of the main passage but to provide separately a bypass passage along the flowing direction of the main passage wherein the cross-sectional area of the bypass passage is changed.
A Karman vortex flow meter having such a bypass passage is disclosed in, for instance, Japanese Unexamined Utility Model Application No. 53422/1980.
The Karman vortex flow meter disclosed in the Publication is shown in FIGS. 1 through 3.
FIGS. 1 and 3 are respectively side views cross-sectioned and a front view of a first embodiment of the Karman vortex flow meter wherein a main passage 1 and bypass passages 2 are provided. The main passage 1 is of a tubular form in which an inlet portion 1a is enlarged; the cross-sectional surface area of the flow passage of the main passage 1 is reduced at a throttle portion 1b, and a reduced cross-sectional surface area portion is extended to an outlet portion 1c. In the outlet portion 1c, a vortex flow generating means 3 and a vortex flow detecting means 4, which constitute a vortex flow detecting device, are provided. The vortex flow detecting means 4 is constituted by an ultrasonic wave transmitter 4a and an ultrasonic wave receiver 4b.
A rectifier 5 is disposed at the enlarged portion, i.e., the inlet portion 1a. An ultrasonic wave absorbing material 6 made of, for instance, non-woven cloth or the like is disposed in the entire region or a necessary portion along the tubular inner wall of the main passage 1 so that the function of the vortex flow detecting means 4 can be improved. The ultrasonic wave transmitter 4a and the ultrasonic wave receiver 4b are respectively covered with net-like bodies 7. The bypass passages 2 are so arranged as to surround the main passage 1 along the flow direction of fluid. The cross-sectional surface area of each of the bypass passages 2 is reduced at an inlet portion 2a and the cross-sectional surface area is enlarged at the portion corresponding to the throttle portion 1b of the main passage 1, the enlarged portion being extended to an outlet portion 2b. Supporting pieces 8 are provided in the bypass passages 2 in order to reinforce the main passage 1 and the bypass passages 2.
FIG. 2 shows a second embodiment of the Karman vortex flow meter disclosed in the above-mentioned Publication. In this embodiment, the bypass passages 2 do not surround the main passage 1 but they are formed separately from the main passage 1 so that the main passage 1 and the bypass passages 2 are joined in the radial direction. The bypass passages 2 have respectively an enlarged inlet portion 2a in the same manner as the enlarged inlet portion 1a of the main passage 1, and they are joined at the inlet portions 1a and 2a. Numerals 9 designate flanges for connecting another passage, and numerals 21 designate rod-like bodies which may have different cross-sectional surface areas so that the flow rate of fluid flowing in the bypass passages can be controlled. The other structural elements are the same as those in the first embodiment.
In either case, the cross-sectional surface area of the main passage 1 is enlarged at the inlet portion 1a and a rectifier 5 is disposed in the enlarged portion so that the detection of a vortex flow can be effectively conducted, and the outlet portion 1c of the main passage 1 in which the flow rate detecting device is disposed has a reduced cross-sectional surface area.
In operation of the conventional vortex flow meters, fluid such as air flowing through a suction pipe (not shown) is introduced through the inlet portions 1a, 2a of the main passage 1 and the bypass passages 2 of the Karman vortex flow meter. The fluid in the main passage 1 is rectified by the rectifier 5, and then, the fluid impinges the vortex generating means 3 in the outlet portion 1c to generate a vortex. The flow rate of the fluid is measured by detecting a quantity of the vortex flow by means of the vortex flow detecting means 4.
The conventional Karman vortex flow meter had the problems described blow.
In the Karman vortex flow meter shown in FIGS. 1 and 3, since a portion extended to the outlet portion 2b from the inlet portion 2a in the bypass passages was abruptly enlarged, there was a large possibility of causing separation and disturbance in the fluid due to a vortex flow at the abruptly enlarged portion. The separation and the disturbance of the fluid resulted in that the kinetic energy of the fluid was transformed into a heat energy, whereby the reduction of the kinetic energy caused an increased pressure loss. When the pressure loss increased, the density of the fluid is decreased. When such a Karman vortex flow meter is disposed in an engine system, the reduction of engine power is invited.
In the Karman vortex flow meter shown in FIG. 2, since the bypass passages 2 were enlarged at each of the inlet portions 2a, the disturbance of the fluid could be minimized at a portion extended to the outlet portions 2b from the inlet portions 2a. However, the conventional flow meter had such a construction that the main passage 1 was formed separately from the bypass passages 2 and then, they were joined later. Accordingly, the wall thickness at a joining portion was fairly thick, so that there was a resistance to the fluid entering into the Karman vortex flow meter, with the result that the disturbance of the fluid was large, and there was an increased pressure loss in the same manner as the flow meter as shown in FIG. 1 and 3.
Further, in the flow meter shown in FIG. 2, since the flow passage of the bypass passages 2 was abruptly enlarged at the outlet portions, the pressure loss was further increased.